Hierarchy neighbors

[adzcai]

A new feature for the Hierarchy Notes alternative hierarchy: e.g. given the following hierarchy note:

---
BC-hierarchy-note-next: next
BC-hierarchy-note-prev: prev
---
- [[A]]
	- [[AA]]
	- [[AB]]
		- [[ABA]]
		- [[ABB]]
	- [[AC]]
		- [[ACA]]
		- [[ACB]]
	- [[AD]]

Then from AB, the "next" field points to AC and "prev" points in the opposite direction; from ABA, the next field points to ABB and "prev" points in the opposite direction; etc.

This was requested in a few different issues:
Closes #392
Closes #327
Closes #476

[SkepticMystic]

Hey @adzcai, thank you for your contribution, I appreciate the effort that's gone into this!
I'm not sure if you've seen, but I'm working on a full rewrite of the plugin, more info here: https://github.com/SkepticMystic/breadcrumbs/blob/master/V4.md. Most features have already been reimplemented, including Hierarchy Notes (now called List Notes).

I'm considering whether to merge your changes into the existing v3, or if they should go into v4 only.. Would you be open to migrating your changes to v4 instead? The code is different, but should be simpler to understand. Let me know your thoughts